1. Field of the Invention
The present invention relates, in general, to a method of measuring an absolute lung volume based on O2/CO2 gas analysis and, more particularly, to a method of measuring an absolute lung volume based on O2/CO2 gas analysis, in which concentrations of oxygen and carbon dioxide gases are measured and analyzed in respiratory gas, consisting of nitrogen (N2), oxygen (O2), and carbon dioxide (CO2), to indirectly measure a concentration of nitrogen, thereby achieving measurement of the absolute lung volume, including a functional residual capacity.
2. Description of the Related Art
Exhaled through expiration during respiration of humans, respiratory gas consists of only nitrogen (N2), oxygen (O2), and carbon dioxide (CO2). A concentration of the respiratory gas exhaled during the respiration of humans is important to evaluate a respiratory function. For example, the evaluation of the respiratory function is implemented in such a way that expired air, which is generated through a physiological gas exchange process in the lung after inspiring 100% oxygen, is collected and subjected to concentration analysis so as to measure, for example, concentrations of oxygen, carbon dioxide, and nitrogen. Used to evaluate the respiratory function, an absolute lung volume, such as a functional residual capacity (hereinafter, referred to as “FRC”), is a very important parameter for diagnosing a chronic respiratory disease.
The FRC denotes a volume of air remaining in the lung at the moment that expiration ends, and is normally about 1-2 L. Of methods of measuring the FRC, a whole-body plethysmography is the most precise method. However, the whole-body plethysmography is disadvantageous in that a volume of air existing in the abdominal cavity is included in measurements, and that it is impossible to apply to claustrophobia patients or patients who have difficulty in breathing because a subject must respire in a closed large box.
Accordingly, an N2 wash-out test is clinically used to measure the FRC. The N2 wash-out test employs the property that nitrogen does not diffuse into a closed capillary. As shown in FIG. 1, in the N2 wash-out test, a three-way valve 50 is connected to a subject 40, who inspires air, to form an air feeding path 13, and an inspiration path 11 and an expiration path 12 of the subject. The inspiration path 11 and the expiration path 12 of the subject 40 are divided by one-way valves 10, 20, and an airbag 30 is connected to the expiration path 12 to collect expired gases.
When the subject 40, who respires air fed through the air feeding path 13, is in an FRC state, that is, when the subject 40 is in the last stage of expiration, the lung of the subject 40 is fully filled with air and a concentration of nitrogen is 79% in the air. Hence, the total nitrogen volume (VN2) of the lung of the subject is 0.79×FRC as shown in Equation 1.VN2=0.79×FRC  Formula 1
At this stage, the three-way valve 50 is adjusted to allow the subject to inspire 100% oxygen fed through the inspiration path 11, and expired gas is collected through the expiration path 12 in the airbag 30. When 100% oxygen is inspired through the inspiration path 13, air in the lung of the subject 40 is diluted by oxygen fed into the lung. If the subject expires the air, which is diluted by oxygen, from the lung, the concentration of nitrogen in the expired gas is reduced to be less than 79% of the initial concentration. Upon repeating this process, the lung of the subject 40 is fully filled with oxygen, and the concentration of nitrogen in the expired gas becomes 0%.
In the FRC state in which the lung of the subject is fully filled with air containing 79% nitrogen, the inspiration of 100% oxygen is repeated to collect the expired gas in the airbag 30 until the concentration of nitrogen is 0% in the expired gas, thereby completely transferring nitrogen from the lung of the subject 40 into the airbag.
Calculated using a volume (VB) of the airbag and a nitrogen concentration ratio (FN2) of gas in the airbag, a volume (VN2) of nitrogen in the airbag 30 is VB×FN2 as shown in Equation 2.VN2=FN2VB  Formula 2
In this regard, since the volume (VN2) of nitrogen in the airbag 30 of Equation 2 must be the same as the volume (VN2) of nitrogen in the lung of Equation 1, the FRC of Equation 3 is calculated using Equations 1 and 2.FRC=FN2VB/0.79  Formula 3
However, the concentration of nitrogen must be measured using a nitrogen concentration analyzer during the N2 wash-out test. The nitrogen concentration analyzer has disadvantages of a high price, noise, and additional use of a large-sized vacuum pump, and thus, its usage is complicated. Accordingly, it is impossible to conduct the N2 wash-out test to evaluate the FRC except in a large hospital.